Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to alternating current voltage rectifiers. More specifically, it relates to a method and apparatus for rectifying a bipolar voltage signal using a current driven, zero-voltage switched, synchronous rectifier.
2. Related Art
Alternating Current (AC) adapters used in supplying power to electronic devices accomplish several tasks. First, and perhaps most importantly, these adapters transform voltage from a high level (e.g., 120V) to a lower, usable level (e.g., 3V, 6V, 12V, 24V). Second, these adapters convert alternating current into direct current (DC). In many cases, small portable electronic devices are designed to operate using a direct current (DC) voltage source. The transformation of a high voltage AC signal to a lower voltage DC signal allows these devices to use power from standard wall outlets. AC adapters generally include a transformer to reduce the voltage level and a rectifier to convert AC signals to DC signals.
Many portable electronic devices are configured to operate under battery power and alternatively under power from an AC adapter. In many of these designs, the output voltage from the AC adapter is used to not only power the electronic device, but also to recharge batteries. In fact, many devices today allow for fast recharging of batteries even while the device is operating. As a result, AC adapters are increasingly subjected to heavy power demands.
The rectification performed by an AC adapter involves the conversion of bipolar (alternately positive and negative) signals into unipolar (never negative) signals. Conventional rectifiers generally use diodes or voltage driven power transistors to perform the rectification. In both cases, increased power requirements have led to increased power dissipation in the rectifying components of the AC adapter.
In conventional AC adapters that use diodes to rectify the voltage, some of the most widely used diodes are PN junction diodes and Schottky diodes. PN junction diodes are minority carrier devices which operate on the principle that a forward-biased diode will permit current flow via minority carrier diffusion, yet will largely prevent current flow (up to a breakdown point) under reverse-bias because carriers are unable to traverse the transition region between the p and n regions. Power dissipation may be a problem with using a PN junction as a rectifier. As a voltage signal goes negative, the diode must dissipate the stored charge that develops during forward-bias. This stored charge is in the form of excess minority carriers which must be depleted by recombination within the diode and by reverse current flowing out of the diode. Ultra-fast recovery PN junction diode designs may be used to help alleviate some of the switching delays and power losses associated with the recombination process. However, in some of the more power-demanding applications, such as portable computers, the AC adapter must provide upwards of 15 to 30 volts DC to the unit. These increased voltage demands require the use of higher voltage capacity diodes. These higher voltage diodes require thicker junction regions, that lead to higher resistivity, a higher volume of stored charge, and a longer recombination time.
Schottky diodes have been used in an effort to alleviate some of the problems associated with using PN junction diodes as rectifiers. Schottky diodes are manufactured with a metal deposited on a semiconductor material. The metal has a larger work function than the semiconductor material on which it is deposited that creates an energy barrier to current carriers attempting to cross the junction between the materials. The energy barrier height is reduced in forward bias thereby allowing majority carriers to cross the junction between the two surfaces and to create a current flow. In reverse bias, the barrier height is increased, and current flow is limited. The key difference between Schottky diodes and PN junction diodes is that the former rely on majority carriers to create current flow while the latter depends on minority carriers for current flow. Since the Schottky diode does not require recombination of minority carriers during transitions from forward to reverse current flow, switching times and switching losses are reduced. Schottky diodes also offer the advantage of providing lower conduction losses than a PN junction diode. Despite these advantages, Schottky diodes inherently exhibit a substantially higher junction capacitance which adversely affects reverse recovery characteristics. In some cases, this capacitance may yield losses on the order of those seen in PN junction diode rectifiers.
Another device commonly used in AC voltage rectifier applications is the power MOSFET (metal-oxide-semiconductor field effect transistor). Like the Schottky diodes, MOSFETs are majority carrier devices. MOSFET transistors also exhibit fast switching speeds and reduced conduction losses. The increased switching speed decreases losses due to reverse current flow in the rectifier. The net result is that power dissipation may be lowered when a MOSFET is used as a rectifier. Some conventional systems use an integrated circuit (IC) to switch the MOSFETs. This aids in controlling the timing of the rectifier circuit, but adds significantly to the complexity. Other conventional AC adapters that use MOSFET switching devices are voltage driven from the primary side of the main voltage transformer. This type of design is inherently inefficient because the switch control signals are delivered across the transformer, which induces delays in the pulse waveforms. The problem is exacerbated by safety requirements for transformers which call for insulation between transformer windings. This separation results in poor coupling and increased leakage inductance in the transformer. In all these cases, a problem with using MOSFETs as voltage rectifiers occurs during light- to no-load conditions, where their switching losses dominate the power loss in the AC adapter.
Requests to promote efficiency improvements of electrical end-use equipment have been promulgated by the Directorate General for Energy (DG-Energy) of the European Commission. These requests include the improvement of no-load efficiency. The xe2x80x9cCode of Conduct on Efficiency of External Power Suppliesxe2x80x9d has set forth a requirement that no-load AC adapter power consumption be restricted to less than 1 Watt by the year 2001. Stricter requirements are requested for subsequent years. The requirements of the Code of Conduct are well known to those skilled in the art and are herein incorporated by reference.
It is desirable therefore, to develop an AC adapter that reduces the no-load power consumption to conform to the European Commission Code of Conduct while offering better efficiency and thermal performance at rated load current. The improved efficiency of the AC adapter may advantageously reduce power consumption and reduce heat dissipation. Furthermore, the AC adapter output rectifier would preferably be self-driven from the secondary side of the transformer so as to decrease device complexity and improve rectifier synchronization. As a result of these benefits, the size of the AC adapter may advantageously be reduced.
The problems noted above are solved in large part by an output rectifier circuit for an AC adapter comprising a current transformer configured to receive and rectify a bipolar signal. The current transformer and the rest of the rectifier circuitry are located on the secondary (load) side of the main transformer. The load current is used to drive the current transformer. The current transformer is comprised of a primary coil and preferably two secondary coils with opposite polarities. A diode is coupled to each of the two secondary coils to allow only positive current flow through the secondary coils. Since the polarities of the secondary coils are opposite one another, current will generally flow through one diode or the other at a given time. The rectifier also includes a rectifying transistor coupled to one of the secondary coils in the current transformer. Positive current from the first secondary coil causes the rectifying transistor to turn on thereby allowing current to flow through the transistor from the input of the rectifier to the output of the rectifier. A pull-down transistor is coupled to the second secondary coil of the current transformer. Positive current from the second secondary coil causes the pull-down transistor to turn on. The pull-down transistor is configured to connect the gate of the rectifying transistor to ground (thereby turning the rectifying transistor off) when the pull-down transistor is turned on. Switching losses inherent in a transistor switched rectifier are reduced by including a capacitor coupled in parallel with the gate of the pull-down transistor which delays switching of the rectifying transistor, thereby permitting zero voltage switching of the rectifying transistor. Zero voltage switching implies that the voltage and current in the switching transistor are not both positive at the same time. The rectifier further comprises a hold-down transistor configured to ground the gate terminal of the pull-down transistor, thereby holding the pull-down transistor off, when the rectifying transistor is on. The hold-down transistor keeps the pull-down transistor from inadvertently turning on (and thereby turning the rectifying transistor off) when the rectifying transistor should be on. Zener diodes are placed in parallel with the gate terminals of the pull-down and rectifying transistors to regulate voltage and prevent damage to the transistors. The above described features of the rectifier improve electrical and thermal efficiency of the AC adapter and may advantageously permit a reduction in the overall size of the AC adapter.